If you’re looking forward to lots of outings with your partner, check out these details about year-round temperatures in Heriot Bay: The hottest months are usually August and July with an average temp of 60°F, while the coldest months are February and December with an average of 36°F. The rainiest months in Heriot Bay are November, January, December, and October, with each month seeing an average of 12 inches of rainfall.